This invention is related to motor vehicle brake control systems and particularly to an improved traction control system incorporating features to reduce the likelihood of developing adverse brake conditions through operation of traction control.
Many motor vehicles today incorporate traction control systems. These systems enhance the directional stability of a vehicle and the total traction availability in conditions where one or more of the drive wheels encounters a low coefficient of friction surface or otherwise develops a wheel spinning condition. The tractive effort available from the spinning wheel is decreased due to the spinning condition. Moreover, most motor vehicles incorporate a differential in which torque applied to the non-spinning wheel on one side of the vehicle is reduced when the drive wheel on the other side encounters a spinning condition. Therefore, total tractive effort is reduced. With traction control, the speeds of the drive wheels are detected and compared with the other wheels on the vehicle. In the case of a vehicle that is also equipped with an anti-lock brake system (ABS), this comparison can be made with all of the wheels on the vehicle. To remedy a wheel spinning condition, the traction control system may reduce engine torque through a control link to the engine controller. In addition, such systems typically also activate the service brake of the spinning wheel, gently pumping the brake to slow the spinning wheel speed. This brake application to the spinning wheel allows the drive differential to transmit higher torque to other drive wheels. Such traction control systems are found in passenger cars having hydraulic braking systems as well as heavy duty trucks employing air brake systems.
In some driving conditions a traction control system can be operating for considerable period of time. For example, in the case of a heavy duty truck climbing a long grade in low traction conditions, a traction control system may activate a service brake on a driving wheel over repeated cycles. However, service braking systems have inherent energy dissipation limits. Accordingly, in such conditions, brake friction material temperatures, and rotor or drum temperatures can reach extreme levels. Since braking torque decreases in such conditions, a condition referred to as brake "fade" occurs and thus continued traction control performance decreases. More importantly however, since time is necessary for the brake parts to cool, braking performance could be degraded and excessive friction material wear occurs. In air brake systems, brake application uses high pressure air stored in a reservoir which becomes depleted through repeated brake application over a short duration. In view of this, it may further be desirable to disable traction control to preserve braking air pressure.
As one means of avoiding the above referenced condition, brake component temperature measurements could be taken. A thermocouple or other temperature sensor can be embedded within a brake lining or other brake part. Through the use of an associated controller, excessively high brake temperatures can be monitored and could also be used to disable a traction control system to preclude brake fade conditions. Alternatively, direct brake pressure measurements could be taken and used to disable traction control in some conditions. Although such systems are believed operable, they would require a significant addition of hardware to a vehicle and would complicate assembly and servicing. In addition, maintenance and reliability problems could be presented.
In view of the foregoing, there is a need for an improved traction control system and method of operating such a system which would reduce the likelihood of adverse brake conditions occurring through operation of traction control.
Since traction control systems require wheel speed inputs and active brake controllers, they are almost universally provided on vehicles also having ABS. In fact, in many cases traction control is provided as an enhancement to an ABS with little or no additional hardware. Through appropriate control algorithms and software, an ABS controller can be operated to provide traction control.
In accordance with the present invention, the development of adverse brake conditions due to traction control operation is prevented through a system and control method which monitors the activity of the traction control system and using assumptions about brake system characteristics, disables the traction control system in some operating conditions.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.